The present invention relates to a coating composition, preferably a printing ink for security applications, to a method for producing a coating composition and to the use of glass ceramics according to the preambles of the independent claims.
Pigments which have luminescent properties (phosphors) are well known and are widely used as marking materials in security applications. Luminescent materials can absorb certain types of energy acting upon them and subsequently emit this absorbed energy as electromagnetic radiation. Down-converting luminescent materials absorb electromagnetic radiation at a higher frequency (shorter wavelength) and re-emit it at a lower frequency (longer wavelength). Up-converting luminescent materials absorb electromagnetic radiation at a lower frequency and re-emit part of it at a higher frequency. Luminescent materials are used for coding and marking of mass-produced goods, high value branded articles and security documents. In certain cases an up-converting luminescent is added as a hidden xe2x80x9ctaggantxe2x80x9d to a transparent or colorless coating composition or printing ink, which is applied to branded goods in form of barcodes, company emblems, labels, etc. This allows a subsequent recognition of the genuine article in the fight against counterfeiters and product piracy.
Light emission of luminescent materials arises from excited states in atoms or molecules. The radiative decay of such excited states has a characteristic decay time, which depends on the material and can range from 10xe2x88x929 seconds up to various hours. Short-lived luminescent emission is usually called fluorescence, whereas long-lived emission is called phosphorescence. Materials of either type of emission are suitable for the realisation of machine-readable codes. Machine-readability is a necessary prerequisite for mass treatment of goods, e.g. in automated production, sorting, quality control, packaging or authentication operations. Machine-verification is also applied outside production or logistic chains for counterfeit or fraud detection.
The common up-converting materials are of inorganic nature and consist essentially of a crystal lattice in which rare-earth ions are present as activators and sensitizers. The excitation and emission characteristics of up-converting materials are inherent characteristics of the rare earth ions employed. Their corresponding optical absorption and emission processes are due to electron transitions within the incompletely filled 4f shell of the rare earth ion. This electron shell is strongly shielded from the chemical environment of the atom, such that variations in the crystal lattice, thermal vibrations, etc. have only a marginal influence on it. Consequently, rare-earth ions have narrow band optical absorption and emission spectra, which are to a great extent independent of the nature of the crystal lattice. The sharp, discrete bands and the low interaction with the crystal lattice usually result in a high saturation of the luminescence color and a high luminescence quantum yield.
Rare-earth ion luminescence activators have relatively long-lived excited states and a particular electronic structure. This permits the energy of two or more photons in succession to be transmitted to one single luminescence centre and cumulated there. An electron is thus promoted to a higher energy level than that corresponding to the incoming photon energy. When this electron returns from its higher level to the ground state, a photon having about the sum of the energies of the cumulated exciting photons is emitted. In this way it is possible to convert e.g. IR radiation into visible light. Alkali and alkaline earth metal halides, and the halides, oxyhalides and oxysulfides of yttrium, lanthanum and gadolinium are principally used as the host material, whereas e.g. Er3+, Ho3+ and Tm3+ serve as the activators. Additionally, ytterbium(3+) and/or other ions can be present in the crystal lattice as sensitizers to increase the quantum yield.
Down-converting luminescents are either of inorganic or of organic (molecular) nature. Irradiation of the luminescent with short-wave light promotes an electron to a higher excited state. Decay of this higher excited state usually follows a cascade to next-lower excited states, and finally to the ground state, and produces light emissions having longer wavelength than the exciting radiation. Typical down-converting luminescents convert UV to visible light. Conversion of UV or visible light to IR, or of lower wavelength IR to higher wavelength IR is also possible. Usually up-converting luminescents can also be exploited in down-converting modes.
However a lot of up-and down-converting materials are not stable when exposed to oxygen, humidity, and, in particular, to organic solvents and/or media containing chemical oxidising or reducing agents. Thus the choice of luminescent materials, particularly of up-converters which are suitable for being blended as pigments into polymer compositions, such as coating composition or printing inks, is limited to only a few types of host crystals.
GB 2 258 659 and GB 2 258 660 describe up-converting materials based on yttrium oxysulfide (Y2O2S), doped with erbium and ytterbium. Further disclosed is the use of such materials as pigments in printing inks for security applications.
Since compositions, synthesis and absorption/emission properties of the common up- and down-converting materials fulfilling the necessary stability criteria are more and more known to counterfeiters as well, there is a constant need for new up-and down-converting materials, having uncommon composition and properties, such as particular luminescence decay characteristics, and/or particular luminescence efficiency and/or, in its case, particular branching ratios between multiple emission possibilities, all of them being exploitable for security purposes.
It is an object of the present invention to overcome the drawbacks of the prior art.
Particularly it is an object of the invention to provide new luminescent pigments, especially those having unusual excitation/emission characteristics. It is a further object of the invention to provide up- and down-converting pigments which are resistant to environmental influences, particularly against organic resins and/or solvents.
These objects are solved by the features of the independent claims. Particularly they are solved by a coating composition, preferably a printing ink for security applications, comprising at least one organic resin, at least one pigment and optionally at least one solvent, characterised in that said pigment comprises glass ceramic particles which contain at least one crystalline phase embedded in a glass matrix, said pigment having a particle size in the range of between 0.1 xcexcm to 50 xcexcm.
Preferably the glass ceramic particles have a particle size in range of between 1 xcexcm to 20 xcexcm and even more preferably in the range of between 3 xcexcm to 10 xcexcm.
Glass ceramics are composite solids, which are formed by controlled devitrification of glasses. (See Rxc3x6mpp Chemie Lexikon, ed. J. Felbe, M. Regitz, 9th edition 1990, page 156.) They can be manufactured by heating (tempering) suitable precursor glasses to allow for partial crystallisation of part of the glass composition. Glass ceramics comprise thus a certain amount of a crystalline phase, embedded in a surrounding glass phase.